In an electrostatographic copy machine, an electrostatic latent image is formed on an element. That image is developed by the application of an oppositely charged toner to the element. The image-forming toner on the element is then transferred to a receiver where it is permanently fixed, typically, by heat fusion. The transfer of the toner to the receiver is usually accomplished electrostatically by means of an electrostatic bias between the receiver and the element.
In order to produce copies of very high resolution and low granularity, it is necessary to use toner particles that have a very small particle size, i.e., less than about 8 micrometers. (Particle size herein refers to mean volume weighted diameter as measured by conventional diameter measuring devices such as a Coulter Multisizer, sold by Coulter, Inc. Mean volume weighted diameter is the sum of the mass of each particle times the diameter of a spherical particle of equal mass and density, divided by total particle mass.) However, it has been found that it is very difficult to electrostatically transfer such fine toner particles from the element to the receiver, especially when they are less than 6 micrometers in diameter. That is, fine toner particles frequently do not transfer from the element with reasonable efficiency. Moreover, those particles which do transfer frequently fail to transfer to a position on the receiver that is directly opposite their position on the element, but rather, under the influence of coulombic forces, tend to scatter, thus lowering the resolution of the transferred image and increasing the grain and mottle. Thus, high resolution images of low granularity require very small particles, however, images having high resolution and low granularity have not been attainable using electrostatically assisted transfer.
In order to avoid this problem, it has become necessary to transfer the toner from the element to the receiver by non-electrostatic processes. One such process is the thermally assisted transfer process where the receiver is heated, typically to about 60 to about 90.degree. C., and is pressed against the toner particles on the element. The heated receiver sinters the toner particles causing them to stick to each other and to the receiver thereby effecting the transfer of the toner from the element to the receiver. The element and receiver are then separated and the toner image is fixed, e.g., thermally fused to the receiver. For details, see U.S. Pat. No. 4,927,727 to Rimai et al.
While the thermally assisted transfer process does transfer very small particles without the scattering that occurs with electrostatic transfer processes, it is sometimes difficult to transfer all of the toner particles by this process. The toner particles that are directly on the element often experience a greater attractive force to the element than they do to the receiver and to other toner particles that are stacked above them, and the heat from the receiver may have diminished to such an extent by the time it reaches the toner particles next to the element that it does not sinter them. As a result, the toner particles that are in contact with the element may not transfer. Attempts to solve this problem by coating the element with a release agent have not proven to be successful because the process tends to wipe the release agent off the element into the developer which degrades both the developer and the development process. Moreover, because the process tends to wipe the release agent off the element, the application of additional release agent to the element is periodically required in order to prevent the toner particles from adhering to the element during transfer.
An alternative approach utilized in the past for removing all of the toner particles from the element was to use a receiver that had been coated with a thermoplastic polymer. During transfer, the toner particles adhered to or became partially or slightly embedded in the thermoplastic polymer coating and were thereby removed from the element. However, it was found that many thermoplastics that were capable of removing all of the toner particles also tended to adhere to the element. This, of course, not only seriously impaired image quality but it also had the potential of damaging both the element and the receiver. Moreover, it was not possible to predict with any degree of certainty which thermoplastic polymers would remove all of the toner particles from the element without sticking to the element during transfer and subsequent separation of the receiver from the element and which ones would not.
Efforts to overcome these problems first focused on applying a layer of a release agent to the surface of the thermoplastic polymer coating on the receiver substrate and heating the receiver above the Tg of the thermoplastic polymer during transfer as described in U.S. Pat. No. 4,968,578 to Light et al. The release agent prevented the thermoplastic polymer coating from adhering to the element, but it would not prevent the toner from transferring to the thermoplastic polymer coating on the receiver and virtually all of the toner was transferred to the receiver. This constituted a significant advancement in the art because it was now possible not only to obtain the high image quality that was not previously attainable when very small toner particles were transferred electrostatically but, in addition, the problem of incomplete transfer was avoided. In addition, several other advantages were provided by this process. One such advantage was that copies made by this process could be given a more uniform gloss because all of the receiver was coated with a thermoplastic polymer, (which could be made glossy) while, in receivers that were not coated with a thermoplastic polymer, only those portions of the receiver that were covered with toner could be made glossy and the level of gloss varied with the amount of toner. Another advantage of the process was that when the toner was fixed, it was driven more or less intact into the thermoplastic polymer coating rather than being flattened and spread out over the receiver. This also resulted in a higher resolution image and less grain. Finally, in images made using this process, light tended to reflect from behind the embedded toner particles that were in the thermoplastic layer which caused the light to diffuse more making the image appear less grainy.
For all of the benefits and advantages provided by this process, however, the application of a release agent to the thermoplastic polymer coating on the receiver in order to prevent the thermoplastic polymer coating from adhering to the surface of the element during transfer and subsequent separation of the receiver from the element created several problems. One such problem was that the release agent tended to transfer to and build up on the element or photoconductor thereby degrading image quality and causing potential damage to both the element and the receiver. Another problem was that the release agent tended to allow the thermoplastic polymer coating to separate from the support or substrate, especially during or after finishing, due to a reduction in the adhesion strength of the thermoplastic polymer coating to the receiver support caused by the tendency of the release agent, which had a lower surface energy than the thermoplastic polymer coating and hence a lesser predilection to adhere to the receiver support than the thermoplastic polymer coating, to migrate through the thermoplastic polymer coating to the interfacial region between the thermoplastic polymer coating and the support and to cause the thermoplastic polymer coating to separate from the support. It was also found that the release agent reduced the gloss of the finished image. Finally, the addition of a release agent to the thermoplastic polymer coating added to the overall cost of the process.
Recently, a technique was described in U.S. Pat No. 5,043,242 to Light et al for obviating the foregoing limitations whereby fine toner particles having a particle size of 8 micrometers or less could be transferred from the surface of an element to a thermoplastic coated receiver with virtually 100% toner transfer efficiency using the thermally assisted method of transfer without having to apply a coating or a layer of a release agent to the toner contacting surface of the thermoplastic polymer coating on the receiver substrate prior to toner transfer in order to prevent the thermoplastic polymer coating from sticking or adhering to the element surface during transfer of the toner particles from the element to the thermoplastic polymer coated receiver and during the subsequent separation of the receiver from the element. Studies revealed that by carefully selecting, as the thermoplastic polymer coated receiver, a receiver in which the thermoplastic polymer coating material was a thermoplastic addition polymer which had a glass transition temperature that was less than approximately 10.degree. C. above the glass transition temperature of the toner binder and the surface energy of the thermoplastic polymer coating was within a range of from approximately 38 to 43 dynes/cm and, as the element on which the toner particles which were to be transferred to the receiver were carried, an element, which had a surface layer which comprised a film-forming, electrically insulating polyester or polycarbonate thermoplastic polymeric binder resin matrix and had a surface energy not exceeding approximately 47 dynes/cm, preferably 40 to 45 dynes/cm, and further, by heating the receiver to a temperature such that the temperature of the thermoplastic polymer coating on the receiver substrate during transfer was at least approximately 15.degree. C. above the Tg of the thermoplastic polymer, it was possible to transfer such very small, fine toner particles (i.e., toner particles having a particle size of less than 8 micrometers) non-electrostatically from the surface of the element to the thermoplastic coated receiver and to obtain high resolution transferred images which were not previously attainable when such small toner particles were transferred electrostatically while at the same time avoiding the problems of incomplete transfer and adherence of the thermoplastic polymer coating to the element during toner transfer in the absence of a layer of a release agent on the thermoplastic polymer coating, i.e., without having to apply a coating or layer of a release agent to the toner contacting surface of the thermoplastic polymer coating on the receiver substrate prior to contacting the thermoplastic polymer coating with the toner particles on the element surface and transference of the particles to the receiver. Furthermore, it was found that by maintaining the temperature of the receiver such that the temperature of the thermoplastic polymer coating was maintained above the Tg of the thermoplastic polymer immediately after transfer while the receiver was separating from the element surface, the receiver would separate readily and easily from the element, while hot, without the thermoplastic polymer coating adhering to the element surface and without the prior application of a release agent to the thermoplastic polymer coating. In addition, it was further found that all of the other advantages inherent in the use of a thermoplastic polymer coated receiver in a thermally assisted transfer process were preserved by the process including the production of copies having a more uniform gloss and images having a less grainy appearance. And, finally, it was possible for the first time to determine in advance, in a thermally assisted transfer process, which thermoplastic polymers could be used as receiver coating materials which would not only remove virtually all of the toner particles from the element during transfer but, at the same time, would not adhere to the element during transfer and subsequent separation of the receiver from the element and which ones would not.
Unfortunately, this technique requires that both the image-bearing element and the thermoplastic polymer coated receiver exhibit certain limiting ranges of surface energies in order to prevent the thermoplastic polymer coated receiver from sticking to the element during the transfer of the toner particles from the element to the receiver and during the subsequent separation of the receiver from the element. For example, the image-bearing element is specified to exhibit a surface energy of less than approximately 47 dynes/cm, preferably from about 40 to 45 dynes/cm and the thermoplastic polymer coated receiver is further specified to exhibit a surface energy which is in the range of approximately 38 to 43 dynes/cm. Such requirements, of course, limit the amounts and types of materials which can be used to form the surface layer of the image-bearing element and the thermoplastic polymer coating on the receiver. Another drawback with this procedure is that it has been found that in many instances there is a tendency for certain combinations of receivers and image-bearing elements to begin sticking to each other at temperatures which are very near the temperatures at which acceptable transfer first occurs. This is especially true for images which require more than one transfer to the same sheet of thermoplastic receiver, since it has been found that the temperature at which the onset of acceptable transfer occurs for the second and subsequent transfer is several degrees higher than for the first transfer. In practice, it would be very desirable to have at least a 5.degree. C. and, more preferably, at least a 10.degree. C. difference in temperature between the onset of acceptable transfer and the onset of sticking. Thus, there is continued need for combinations of thermoplastic receivers and image-bearing elements exhibiting broader ranges of surface energies which can be used in the practice of the thermally assisted method of transferring small, dry toner particles from the surface of an image-bearing element to a thermoplastic polymer coated receiver which not only will effect the transfer of such small toner particles from the surface of the element to the thermoplastic polymer coated receiver without the thermoplastic polymer coating of the receiver sticking to the element surface during toner transfer in the absence of a layer or a coating of a release agent on the surface of the thermoplastic polymer coating on the receiver or the element, but which also will further expand or increase the range of temperature between the onset of acceptable transfer and the onset of sticking of the image-bearing element to the receiver.